1. Field of the Invention
The present invention relates to an image pickup apparatus for picking up an object image.
2. Related Background Art
FIG. 1 shows an example of the arrangement of a conventional solid-state image pickup element in which pixels having photoelectric conversion units are arranged two-dimensionally. Referring to FIG. 1, a pixel 101 has a photoelectric conversion unit such as a photodiode. A pixel area 100 for picking up an object image is formed by arranging such pixels two-dimensionally.
A signal from a pixel is output to a vertical signal line 103. A holding capacitor 104 temporarily stores the signal output to the vertical signal line 103. A transfer MOS transistor 105 transfers the signal output to the vertical signal line 103 to the holding capacitor 104. A transfer MOS transistor 106 transfers a signal from the holding capacitor 104 to a horizontal signal line 107.
A vertical scanning circuit 108 performs control to read out signals from pixels on each line to the vertical signal line by sequentially scanning pixels one line at a time in the horizontal direction. A horizontal scanning circuit 109 sequentially reads out signals from the holding capacitor 104 to the horizontal signal line 107 by controlling the transfer MOS transistor 106. A reset MOS transistor 110 resets the horizontal signal line. A constant output current source 107 forms a source follower together with the transistor included in the pixel.
The arrangement of color filters of the conventional solid-state image pickup element will be described below. FIG. 2 shows an example of this arrangement, which includes a first color filter 201 that transmits red light, a second color filter 202 that transmits green light, and a third color filter 203 that transmits blue light. In correspondence with the respective pixels arranged two-dimensionally, the first and second color filters are alternately arranged for odd-numbered columns starting from the first column, and the second and third color filters are alternately arranged for even-numbered columns starting from the second column. The second color filters are arranged so as not be adjacent to each other in the horizontal direction in odd- and even-numbered columns.
In general, as shown in FIG. 3, microlenses corresponding to the respective pixels are used to increase the sensitivity of the solid-state image pickup element. FIG. 3 shows a sectional structure of a unit pixel of the solid-state image pickup element on which microlenses are formed. A unit pixel 300 (corresponding to one pixel 101 in FIG. 1 is comprised of a photoelectric conversion unit 301, insulating layer 302, interconnection layers 303 and 304, light-shielding layer 305, passivation layer 306, planarizing layers 307 and 309, color filter layer 308, and microlens 310. Each microlens has a diameter a and a thickness b. This microlens improves the focusing efficiency of incident light to realize high sensitivity.
A general method of manufacturing a microlens will be described next with reference to FIG. 4. The upper layer of a color filter layer 408 is coated with a transparent resin 409 to planarize the surface. The resultant surface is then coated with a microlens material 410 made of an organic resin. The microlens material is patterned by exposure with a mask. Each pattern has a size a′. The microlens material has a thickness b′. As shown in FIG. 4, spaces 411 for isolating the microlenses are formed by development. The resultant structure is fluidized and solidified by heat treatment, thus forming desired microlenses.
As described above, according to the conventional solid-state image pickup element, a plurality of color filters are arranged in the pixel 101, as shown in FIG. 2.
According to this method, however, in a solid-state image pickup element having 640 (horizontal) pixels×480 (vertical) pixels with a pixel pitch of 10 μm, the focal length of each lens for providing a standard field angle is 8 mm, which is equal to the width across corner of the solid-state image pickup element.
If, therefore, an image pickup apparatus such as a digital camera is to be manufactured by using such solid-state image pickup elements, a reduction in thickness of the apparatus is undesirably limited.
A conventional solid-state image pickup element is available, in which microlenses are formed to respective pixels having photodiodes and formed on a single chip, light from an object is focused onto the photodiodes through the respective microlenses, and output signals from the respective pixels are processed by an image processing unit, thereby forming an image.
FIG. 5 is a sectional view showing peripheral portions of a photodiode and microlens of a conventional CCD image pickup element. FIG. 6 shows a substrate 21 made of silicon or the like, a photodiode 22 formed on the substrate 21, an oxide layer 29 formed on the substrate 21 on which the photodiode 22 is formed, a three-layer interconnection 23 made of polysilicon, to which a clock signal for transferring charge converted by the photodiode 22 is transmitted, a light-shielding layer 24 made of tungsten or the like, which mainly shields a charge transfer vertical CCD register VCCD formed blow the interconnection 23 against light, a first passivation layer 25 made of SiO2 or the like and a second passivation layer 30 made of an SiON-based material or the like, which protect the photodiode 22 and the like against the open air (O2, H2O) and impurity ions (K+, Na+) and the like, a planarizing layer 26 made of an organic material and serving to reduce recesses/projections on the second passivation layer 30, and a microlens 27 formed on the planarizing layer 26 and serving to focus light from an object onto the photodiode 22.
In the solid-state image pickup element shown in FIG. 5, the photodiode 22 is formed on the substrate 21 by ion implantation or the like, and the three-layer interconnection 23 is formed with a dielectric interlayer. The light-shielding layer 24 is also formed with the dielectric interlayer. In this case, the light-shielding layer 24 is formed so as to cover the interconnection 23 to prevent light from striking the vertical CCD register VCCD. Subsequently, the first passivation layer 25 is formed on the light-shielding layer 24 to protect the photodiode 22 against the open air and moisture. The second passivation layer 30 is then formed on the first passivation layer 25. Since recesses/projections with a level difference of about 7,000 Å are formed on the second passivation layer 30 in accordance with the shape of the light-shielding layer 24, the planarizing layer 26 is formed to reduce the recesses/projections. More specifically, the upper surface of the second passivation layer 30 is coated with an organic material, and the organic material is planarized by reflowing with heat, thereby forming the planarizing layer 26 with a thickness of about 1 μm with respect to a projection of the second passivation layer 30.
The distance between the upper surface of the planarizing layer 26 formed in this manner and the upper surface of the photodiode 22 is about 4 to 6 μm. The microlens 27 is then formed on the planarizing layer 26, thus manufacturing a solid-state image pickup element like the one shown in FIG. 5.
FIG. 6 is a sectional view showing peripheral portions of a photodiode and microlens of a MOS image pickup element. Referring to FIG. 6, a floating diffusion area 32 is a transfer destination for charge converted by the photodiode 22, a transfer gate 31 controls the transfer of the charge converted by the photodiode 22, and a selective oxide layer 33 is formed for isolation from an adjacent image pickup element.
The same reference numerals as in FIG. 5 denote the same parts in FIG. 6. In this element, a single-layer interconnection 23 is formed, and a light-shielding layer 24 is made of aluminum. The solid-state image pickup element is manufactured by the same procedure as that for the element shown in FIG. 5.
According to the conventional CCD image pickup element, however, the distance from the upper surface of the light-shielding layer to the upper surface of the oxide layer is as large as about 7,000 Å. If, therefore, light is obliquely incident on the microlens, the light is focused on the light-shielding layer instead of the photodiode in some case. If the light is not focused on the photodiode by the microlens, the photosensitivity of the photodiode may deteriorate or light shading may occur.
The conventional MOS image pickup element is larger in pixel size than the CCD image pickup element, and hence allows an increase in the width of one side of a photodiode. Even if, therefore, light is obliquely incident on the microlens, the light is focused on the photodiode. However, with the recent demands for a reduction in the size of pixels of a CMOS sensor, photodiodes smaller in size than conventional ones are being formed. More specifically, the length of each side of a photodiode has been reduced from about 3 μm to 1.5 μm. With a reduction in photodiode size, as in the CCD image pickup element, light is focused on the light-shielding layer by the microlens. This may lead to a deterioration in the photosensitivity of the photodiode and the occurrence of light shading.